System and method for measurement incorporating a crystal oscillator

ABSTRACT

A system, method and device for interrogating a downhole environment in a borehole beneath a surface includes a source of electromagnetic energy, operable to transmit an electromagnetic signal in the borehole, a sensor module, including a passive resonating circuit including a crystal oscillator having a resonant frequency that varies with changes in the condition in the downhole environment to reflect the electromagnetic signal and to modulate the electromagnetic signal in response to a condition in the downhole environment in the borehole and a detector positionable to receive the reflected modulated electromagnetic signal.

BACKGROUND

1. Field

The present invention relates generally to remote sensing and more particularly to sensing temperatures and/or pressures using a crystal oscillator based sensor.

2. Background

In resource recovery, it may be useful to monitor various conditions at locations remote from an observer. In particular, it may be useful to provide for monitoring conditions at or near to the bottom of a borehole that has been drilled either for exploratory or production purposes. Because such boreholes may extend several miles, it is not always practical to provide wired communications systems for such monitoring.

U.S. Pat. No. 6,766,141 (Briles et al) discloses a system for remote down-hole well telemetry. The telemetry communication is used for oil well monitoring and recording instruments located in a vicinity of a bottom of a gas or oil recovery pipe. Modulated reflectance is described for monitoring down-hole conditions.

As described in U.S. Pat. No. 6,766,141, a radio frequency (RF) generator/receiver base station communicates electrically with the pipe. The RF frequency is described as an electromagnetic radiation between 3 Hz and 30 GHz. A down-hole electronics module having a reflecting antenna receives a radiated carrier signal from the RF generator/receiver. An antenna on the electronics module can have a parabolic or other focusing shape. The radiated carrier signal is then reflected in a modulated manner, the modulation being responsive to measurements performed by the electronics module. The reflected, modulated signal is transmitted by the pipe to the surface of the well where it can be detected by the RF generator/receiver.

SUMMARY

An aspect of an embodiment of the present invention includes a source of electromagnetic energy, operable to transmit an electromagnetic signal in the borehole, a sensor module, including a passive resonating circuit including a crystal oscillator having a resonant frequency that varies with changes in the condition in the downhole environment to reflect the electromagnetic signal and to modulate the electromagnetic signal in response to a condition in the downhole environment in the borehole and a detector positionable to receive the reflected modulated electromagnetic signal.

DESCRIPTION OF THE DRAWINGS

Other features described herein will be more readily apparent to those skilled in the art when reading the following detailed description in connection with the accompanying drawings, wherein:

FIG. 1 is a schematic illustration of a system for interrogating a downhole environment in a borehole beneath a surface in accordance with an embodiment of the present invention;

FIG. 2 is a schematic illustration of a sensor package incorporating a pressure or temperature sensor in accordance with an embodiment of the present invention;

FIG. 3 is a schematic illustration of a circuit incorporating a crystal oscillator based sensor in accordance with an embodiment of the present invention;

FIG. 3A is a schematic illustration of a circuit incorporating a crystal oscillator based sensor and a capacitive sensor in accordance with an embodiment of the present invention; and

FIG. 4 is a schematic illustration of a package incorporating a plurality of sensors in accordance with one or more embodiments of the present invention.

DETAILED DESCRIPTION

FIG. 1 illustrates an example of an apparatus 100 for monitoring a condition in a subsurface borehole. The apparatus 100 includes an electromagnetically transmissive medium, such as a conductive line 102, for conducting electromagnetic energy through the borehole. It will be appreciated by those having ordinary skill in that art that the conductive line 102 may take different forms or embodiments, depending on the state of the borehole. Thus, for example, the conductive line 102 may comprise a production tubing string in a completed borehole or a drillstring in a borehole under construction. Near the top of the conductive line 102, a transformer 104 is provided to couple the conductive pipe to a source of electromagnetic energy. Alternate coupling methods to the transformer 104 may be employed. For example, the transmission line may directly couple to a coaxial cable or any other suitable cable.

In the example embodiment as shown, the transformer 104 includes a stack of ferrite rings 106, and a wire 108 wound around the rings. The wire 108 includes leads 110 that may be coupled to a signal generator 112 which may be configured to produce a pulsed or a continuous wave signal, as necessary or desirable. The wire 108 may further be coupled to a receiver 114. The receiver 114 may be embodied as a computer that includes a bus for receiving signals from the apparatus 100 for storage, processing and/or display. In this regard, the computer 114 may be provided with a display 118 which may include, for example, a graphical user interface.

The computer 114 may be programmed to process the modulated frequency to provide a measure of the sensed characteristic. The computer 114 may perform any desired processing of the detected signal including, but not limited to, a statistical (e.g., Fourier) analysis of the modulated vibration frequency, a deconvolution of the signal, a correlation with another signal or the like. Commercial products are readily available and known to those skilled in the art that can be used to perform any suitable frequency detection. Alternately, the computer may be provided with a look-up table in memory or in accessible storage, that correlates received modulated frequencies to sensed acoustic energy.

In a typical drilling application, the borehole will be lined with a borehole casing 120 which is used to provide structural support to the borehole. This casing 120 is frequently made from a conductive material such as steel, in which case it will cooperate with the line 102 in order to form a coaxial transmission line, and it is not necessary to provide any additional conductive medium. Where the casing is not conductive, a conductive sleeve (not shown) may be provided within the casing in order to form the coaxial structure. In order to maintain a spacing between the line 102 and the casing 120, the apparatus 100 may include dielectric rings 122 disposed periodically along the conductive line 102.

The spacers can, for example, be configured as insulated centralizers which can be disks formed from any suitable material including, but not limited to, nylon or polytetrafluoroethylene (PTFE). Though the illustrated embodiment makes use of a coaxial transmission line, it is contemplated that alternate embodiments of a transmission line may be employed, such as a single conductive line, paired conductive lines, or a waveguide. For example, the casing alone may act as a waveguide for certain frequencies of electromagnetic waves. Furthermore, lengths of coaxial cable may be used in all or part of the line. Such coaxial cable may be particularly useful when dielectric fluid cannot be used within the casing 120 (e.g., when saline water or other conductive fluid is present in the casing 120).

A probe portion 124 is located near the distal end of the apparatus 100. In principle, the probe portion may be located at any point along the length of the transmission line. Indeed, multiple such probe portions may be placed at intervals along the length, though this would tend to create additional signal processing burdens in order to differentiate signals from the several probes. Setting a natural resonance frequency of each probe at a different frequency would, in principle, allow for a type of wavelength multiplexing on the coaxial line that could simplify the processing.

The probe portion includes a port 126 that is configured to communicate ambient pressures from fluid present in the borehole into the probe where it may be sensed by the sensor (not shown in FIG. 1). Below the probe is illustrated a packer 128 and packer teeth 130.

In use, the signal generator 112 generates an electromagnetic pulse that is transmitted through the transmission line to the probe 124. In an alternate arrangement, the pulse may be generated locally as described in U.S. patent application Ser. No. 11/898,066, herein incorporated by reference.

The probe includes a sensor that includes a resonant circuit portion that, upon receiving the pulse, modulates and re-emits or reflects the pulse back up the transmission line. The resonant circuit may be, for example, a tank circuit that includes inductive and capacitive components.

In an embodiment, illustrated in FIG. 2, a crystal-based oscillator 200 acts as the L-C tank circuit. The structure of the housing 202 has at one end a pressure feed-in tube 204 that allows pressure from the borehole environment that has entered via the port 126 to pass into an interior space 206 of the sensor 200. In the interior space, the pressure is transmitted to a flexible diaphragm 208 or otherwise pressure-reactive structure.

Motion of the diaphragm 208 is transmitted to a quartz crystal 210, or other piezoelectric crystal such as gallium phosphate. As pressure is transmitted to an edge of the quartz crystal, its resonant frequency changes. By correct selection of a direction of the face of the crystal, the sensor may be made to be more sensitive to pressure or to temperature (e.g., AC-cut). For pressure monitoring, the crystal should be preferentially sensitive to pressure and relatively less sensitive to temperature (e.g., AT-cut). Furthermore, for monitoring of pressure changes with a relatively high frequency response (e.g., monitoring of acoustic frequencies), it is useful for the crystal to be generally relatively thin (e.g., 0.2-2.0 mm) and a typical size is on the order of 1 cm in diameter.

A return spring mechanism 214 may be provided to bias the crystal 210 and its holders towards the feed-in tube 204 and thereby to tend to cause the diaphragm to return to a neutral position. An electrical feed through 216 is provided to couple the sensor 200 to the sensor circuit (not shown).

The sensor 200 may be coupled to the transmission line via an inductive ferrite ring 400 as illustrated in FIG. 3. Electrical leads 402 are provided through the electrical feed through 216 of the sensor module. The leads 402 couple wire loops around the ferrite ring 400. In this embodiment, the oscillator has the characteristics of an L-C circuit and the ferrite ring essentially acts as a transformer to couple the oscillator to the transmission line.

FIG. 3A illustrates an alternate embodiment directed to a pressure sensor configuration. In this embodiment, the relatively temperature-insensitive crystal (e.g., AT cut crystal) is isolated from the ambient pressure, and a capacitive pressure-responsive element 404 is provided in series with the sensor 200′ and exposed to the ambient pressure. In this configuration, the ferrite ring 400 again acts as a transformer, while the capacitive sensor 404 in combination with the crystal sensor 200′ acts as the L-C tank circuit. The crystal sensor 200′ will resonate with a frequency that depends in large part on the capacitance of the capacitive sensor 404. In this case, the capacitive sensor acts to pull the base frequency of the crystal oscillator as a function of the pressure sensed at the capacitor.

FIG. 4 illustrates a package for sensors in accordance with embodiments of the present invention. A number of sensors 500 are disposed within a common housing 502. For each sensor 500, there is a corresponding ferrite ring 400, which is disposed in a portion 504 of the housing 502 that is made from a dielectric material, for example PTFE. While ordinarily there will be a one-to-one ratio of sensors to rings, it is also possible to have one ring correspond to two or even more sensors. As described above, the rings 400 couple the sensors to the transmission line 102. The sensors, in turn, are held in a metal block portion 506 of the sensor module. Tubing 508 is threaded into the metal block in order to positively locate the sensor package. In a typical application, this tubing may constitute either the production tubing itself, or an extension of the production string.

As will be appreciated, it is possible to combine pressure and temperature sensors in a single package, such that the temperature measurements may be used to help account for temperature related drift of the pressure sensor.

To account for variations in response that are well-dependent rather than temperature or pressure dependent, a calibration crystal sensor may be included along with the primary sensor. In this approach, the calibration crystal sensor is provided with its own power source, for example a battery. The resulting sensor is isolated from the well impedance, eliminating well-dependent effects. As an example, the sensor circuitry may include transistors that, in part, act to isolate the calibration crystal sensor when under power. Though the battery may be of limited life, it is possible to use measurements from the calibration crystal sensor during the battery lifetime, and then apply the generated calibration data to ongoing measurements after the calibration sensor has expired. In this regard, a calibration curve or calibration lookup table may be generated over the battery lifetime and stored for use in later measurements.

Another approach is to make use of a temperature insensitive crystal that is isolated from the ambient pressure, similar to that used in the pressure sensor of FIG. 3A. In this variation, the crystal signal, isolated from pressure and relatively insensitive to temperature, will only react to the particular electromagnetic transmission characteristics of the well in which it is positioned. Therefore, its output can be regarded as being representative of the well shift only, unaffected by the other environmental factors. As will be appreciated, this approach may be used in conjunction with the powered calibration sensor previously described to provide additional information regarding the nature of the well-shift phenomenon. In this regard, the powered sensor may be used for calibrating the well-shift monitoring crystal sensor during the period in which the power supply is active. Once the power supply is exhausted, then the unpowered well-shift monitoring crystal sensor may continue to be used in accordance with the previously measured and stored calibration information.

Those skilled in the art will appreciate that the disclosed embodiments described herein are by way of example only, and that numerous variations will exist. The invention is limited only by the claims, which encompass the embodiments described herein as well as variants apparent to those skilled in the art. 

1. A system for interrogating a downhole environment in a borehole beneath a surface, comprising: a source of electromagnetic energy, operable to transmit an electromagnetic signal in the borehole; a sensor module, comprising a passive resonating circuit, the passive resonating circuit comprising a crystal oscillator having a resonant frequency that varies with changes in the condition in the downhole environment to, in use, reflect the electromagnetic signal and to modulate the electromagnetic signal in response to a condition in the downhole environment in the borehole; and a detector positionable to receive the reflected modulated electromagnetic signal.
 2. A system as in claim 1, wherein the sensor module further comprises: a power source; a calibration crystal oscillator, operably communicated to the power source, and configured to modulate a signal from the power source in accordance with the condition in the downhole environment, to produce a calibration signal; a transmission line, in electrical communication with the calibration crystal oscillator, configured and arranged to transmit the calibration signal; and wherein the system further comprises: a processor, configured and arranged to, in use, receive the modulated electromagnetic signal and the calibration signal and, based on the received signals, calculate a calibrated value for the condition in the downhole environment.
 3. A system as in claim 1, wherein the crystal oscillator has a face along a crystal direction such that it is relatively more sensitive to changes in pressure than to changes in temperature.
 4. A system as in claim 1, wherein the crystal oscillator has a face along a crystal direction such that it is relatively more sensitive to changes in temperature than to changes in pressure.
 5. A system as in claim 1, wherein the condition comprises a condition selected from the group consisting of: temperature, pressure and combinations thereof.
 6. A system as in claim 1, wherein the condition is a pressure in the downhole environment, the crystal oscillator is isolated from ambient pressure and the system further comprises a capacitive pressure sensor in series with the crystal oscillator.
 7. A system as in claim 6, wherein the crystal oscillator is mechanically coupled to a pressure-responsive device such that, in use, the pressure in the downhole environment received by the pressure-responsive device is transmitted as a force to an edge of the crystal to change the resonant frequency of the crystal oscillator.
 8. A system as in claim 1, wherein the sensor module comprises a further passive resonating circuit comprising a further crystal oscillator, wherein the crystal oscillator is cut along a crystal direction such that it is relatively more sensitive to changes in temperature than to changes in pressure and the further crystal oscillator is cut along a crystal direction such that it is relatively more sensitive to changes in pressure than to changes in temperature.
 9. A system as in claim 1, wherein the sensor module further comprises a calibration crystal oscillator, substantially isolated from an ambient pressure in the borehole, and configured to produce a calibration signal representative of well-dependent characteristics of the borehole; a processor, configured and arranged to, in use, receive the modulated electromagnetic signal and the calibration signal and, based on the received signals, calculate a calibrated value for the condition in the downhole environment.
 10. A method of interrogating a downhole environment in a borehole beneath a surface, comprising: transmitting an electromagnetic signal in the borehole; reflecting the electromagnetic signal with a sensor module, comprising a passive resonating circuit, the passive resonating circuit comprising a crystal oscillator having a resonant frequency that varies with changes in the condition in the downhole environment; modulating the electromagnetic signal in accordance with the varying resonant frequency in response to the condition in the downhole environment in the borehole; and receiving the reflected modulated electromagnetic signal. 